This is actually a very relative question... A good video processor will outperform the Oppo or most other upconverting DVD player and certain companies have better processing inside of them then others do.
Format conversion (transcoding) is taking one format like composite, s-video, or component, and converting it into another - like HDMI. But, the conversion from 480i material to 480p is indeed upconversion. Yet, how good the Denon does this really needs to be seen in the labs before it is judged.
The Yamaha uses Faroudja processing which is world class stuff and far better than many DVD players have inside of them. Plus, it does the full conversion up to 720p or 1080i which can help with the final quality signal that is delivered to the display.
DVDs actually aren't 480i specifically. There are a combination of 480i and 480p material that is natively encoded on the disc and perhaps the number one issue manufacturers have is that they do a very poor job of deinterlacing. The Panasonic S77 for 200 bucks has a very high reputation for the deinterlacing, then scaling to 720p/1080i. Plus, at 200 bucks or so, it is really a great price.
But, if you have many non-HDMI sources that you are hooking up or you really need the simplicity of a single connection, I would think that the upconversion feature is totally secondary. Look at the HDMI switching, the audio performance, the connectivity, the power capability... etc., etc. Basically - the overall performance and value of the product you are getting.
Either way, for the price, I would get a upconverting DVD player if I needed one, but not go more than about a couple hundred bucks with the new HD disc players coming out soon.